Investigators in the weighing arts seeking to improve the range and accuracy of force measuring devices are called upon to consider and attempt to compensate or to correct for a variety of phenomena. Such phenomena, for example, have been identified as drift, anelastic creep, hysteresis, load position sensitivity, environmental contamination, temperature effects, and the like, and are observed to continue to pose design constraints and operational limitations.
Typically, a force measuring device will be configured having a weight or load platform supported, in turn, by a load cell positioned within a force transfer path extant between loads applied to the platform and the device or scale. The load cell generally is provided as a structure or "counterforce" which is stressed by the application of force within the force transfer of communication path and responds thereto in strain which is instrumented via, for example, strain gauges to provide a weight responsive output.
The phenomenon noted as "creep", as is encountered in the use of load cells, typically is manifested as change, either an increase or decrease, in the output of the load cell with time under an applied load. When the load is removed, the cumulative change is output, i.e. the anelastic creep error, is seen to remain then decrease with time. A hysteresis effect is one associated with the random utilization of weighing devices wherein loads are applied and then removed in any of a myriad of sequences. Where such load and partially unload or reload sequences occur, the outputs or readings of the scales will evidence discrepancies at the same minimal load which are termed a hysteresis effect. Load position sensitivity is an output discrepancy which is occasioned with the positioning of a load at different locations on the scale platform. Such position may, without correction, evolve moments or torques within the load cell structure to adversely affect the output signals therefrom. Environmental contamination generally is accommodated for with some form of encapsulation, i.e. hermetic sealing of the instrumentation of a scale structure. Such protection often is required in view of the industrial environments in which scales typically are employed. The design of such environmental protection often has led to large bulksome structuring or has induced difficulties, inasmuch as the encapsulating structure itself may adversely affect the performance of a load cell structure.
Anelastic creep and hysteresis effects particularly have been associated with load cells utilizing strain gauge based instrumentation. Approaches in avoidance of the phenomena have used piezoelectric and vibrating wire resonator sensors, which have less load bearing capacity than strain gauge based load cells, in conjunction with "load sharing" devices. Such load sharing devices are structured to assert only a lesser but proportionate component of the applied load upon the vibrational or resonant sensor through a transmission beam or its equivalent.
In general, the very rigid resonator based on vibrating wire based sensing approaches pose difficulties in implementation due to their inherent delicacy and the difficulties of their protection with respect to environmental effects and the like.
Another approach to minimizing the hysteresis and creep effects has been to improve the quality of the load cell counterforce material. For example, lower levels of creep or hysteresis are exhibited by forming the counterforces of such materials as beryllium-copper, quartz or glass/ceramic materials. However, to the present time, these approaches are considered overly-expensive for employment with scale structures intended for conventional utilization. Of particular importance, counterforces formed of such somewhat exotic materials are subjected to certain manufacturing difficulties, particularly in terms of severe restrictions on their size and thus their load range capabilities. Generally, where such size limitations are present, the smaller load cells formed of such materials tend to be load position or moment sensitive.
A common load cell counterforce geometry is one incorporating a guided beam or Roberval approach. The guided beam is structured such that, when loaded, it is counterpoised by a moment with a result of no beam end rotation. Conventional Roberval based load cells will be formed as a parallelogramic frame counterforce which is instrumented with strain responsive sensors such as strain gauges. Formed generally from readily fabricable and convenient material such as aluminum, the guided beam based load cells exhibit a desirably improved performance with respect to the avoidance of load position or moment insensitivity.
Investigators have found that additional performance errors, for example hysteresis and creep, theoretically can be cancelled or minimized where the guided beam frame incorporates a load sharing feature as above described, but wherein the transmission beam is employed to impart a proportionate share of the applied load under a condition wherein the average stress within the transmission beam is equated to that in the frame. The controlled stress of the frame or Roberval mechanism typically is established at reduced beam sections, as is the average stress at a correspondingly reduced section of the transmission beam. However, such desired error cancellation has been determined to occur only where the sensor acted upon by the transmission beam, is rigid or theoretically infinitely stiff. The above-noted vibrating wire, for example, has been employed to meet this requirement of sensor rigidity. However, such infinitely stiff sensing mechanisms, when so employed, are accompanied by the above-noted drawbacks.